core.async - Communicating Sequential Processes in Clojure

Talk given at Lambda Lounge on April 8, 2014 http://www.meetup.com/Lambda-Lounge-Utah/events/164368202/

Every language these days seems to be adding some kind of asynchronous support. With all of the available options and buzzwords it is easy to lose sight of what problems we are trying to solve and how these new approaches address all the pain points. We'll begin by motivating the need for asynchronous programming and discuss the approaches typically taken (e.g. callbacks, promises) along with associated limitations and trade-offs. Communicating Sequential Processes (CSP) will then be introduced via Clojure's core.async library and show how this model help reduces the complexity of working with asynchronous events. We will also look at the implementation of core.async, how it adds the functionality as a library via macros and what trade-offs come along with that decision.

Transcript

1. Ben Mabey
   Communicating Sequential Processes in Clojure
   core.async
   @bmabey
   Lambda Lounge Utah, April 8 2014
   

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2. Everything is a Remix
   … but especially this presentation.
   

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3. Everything is a Remix
   … but especially this presentation.
   https://speakerdeck.com/hlship/mastering-time-with-clojure-core-dot-async
   Mastering Time with Clojure core.async
   by Howard M. Lewis Ship
   http://www.infoq.com/presentations/Death-by-Accidental-Complexity
   Death by Accidental Complexity
   by Ulf Wiger
   https://github.com/stuarthalloway/presentations/wiki/Core-Async
   core.async
   by Stuart Halloway
   

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4. Event loop w/Callbacks
   Promises/Futures/Tasks
   Rx (push based FRP)
   Coroutines & Generators
   Actors
   Communicating Sequential Processes (CSP)
   async keyword
   

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5. core.async is an efficient implementation
   of CSP that drastically reduces the
   accidental complexity usually associated
   with concurrent programming.
   

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6. core.async is an efficient implementation
   of CSP that drastically reduces the
   accidental complexity usually associated
   with concurrent programming.
   

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7. Embarrassingly Parallel
   …emparallel? aka brain dead concurrency
   

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8. (defn request-handler!
   [request]!
   (let [data1 (do-query-1 request)!
   data2 (do-query-2 request)!
   data3 (do-query-3 request)]!
   (send-response data1 data2 data3)))
   Embarrassingly Parallel
   …emparallel? aka brain dead concurrency
   

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9. 1 3
   2
   

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12. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    

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13. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    1
    2
    3
    

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14. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    1
    2
    3
    Any value can be placed
    in channel, except for nil
    

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15. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    1
    2
    3
    

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16. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    1
    2
    3
    

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17. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    1
    2
    3
    

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18. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    3
    

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19. Channel
    Writer
    Writer
    Writer
    Reader
    Reader
    

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20. Channel
    Reader
    Reader
    (close! channel)
    

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21. Channel
    Reader
    Reader
    (close! channel)
    nil
    nil
    

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22. Channel
    Writer
    Writer
    Writer
    

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23. Channel
    Writer
    Writer
    Writer
    1
    2
    3
    4
    

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24. Channel
    Writer
    Reader
    Reader
    Writer
    Writer
    1
    2
    3
    4
    

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25. Channel
    Writer
    Reader
    Reader
    Writer
    Writer
    1
    2
    3
    4
    

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26. View Slide

27. (defn request-handler!
    [request]!
    (let [data1 (do-query-1 request)!
    data2 (do-query-2 request)!
    data3 (do-query-3 request)]!
    (send-response data1 data2 data3)))
    

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28. (defn request-handler [request]
    (let [query1 (thread (do-query-1 request))
    query2 (thread (do-query-2 request))
    query3 (thread (do-query-3 request))
    data1 (data2 (data3 ((send-response data1 data2 data3)))
    Evaluate in other thread,
    return channel
    

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29. (defn request-handler [request]
    (let [query1 (thread (do-query-1 request))
    query2 (thread (do-query-2 request))
    query3 (thread (do-query-3 request))
    data1 (data2 (data3 ((send-response data1 data2 data3)))
    block thread until value
    available
    

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30. (thread (do-query-1 request)) (Result – (chan 1)
    (do-query-1
    request)
    

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31. View Slide

32. core.async is an efficient implementation
    of CSP that drastically reduces the
    accidental complexity usually associated
    with concurrent programming.
    

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33. View Slide

34. View Slide

35. (defn request-handler [request]
    (let [query1 (thread (do-query-1 request))
    query2 (thread (do-query-2 request))
    query3 (thread (do-query-3 request))
    data1 (data2 (data3 ((send-response data1 data2 data3)))
    

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36. (defn request-handler [request]
    (let [result (go
    (let [query1 (thread (do-query-1 request))
    query2 (thread (do-query-2 request))
    query3 (thread (do-query-3 request))
    data1 (data2 (data3 ((send-response data1 data2 data3)))]
    (

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37. (go
    (let […

    data1 (data2 (data3 ((send-response data1 data2 data3)))
    Executes in pooled
    thread
    Reads value or parks
    

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38. (go
    (let […

    data1 (data2 (data3 ((send-response data1 data2 data3)))
    Executes in pooled
    thread
    Reads value or parks
    (defn request-handler [request]
    (let [result (go …)]
    (Final result available through
    channel
    

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39. View Slide

40. View Slide

41. (defn request-handler [request]
    (go
    (let [query1 (thread (do-query-1 request))
    query2 (thread (do-query-2 request))
    query3 (thread (do-query-3 request))
    data1 (data2 (data3 ((send-async-response request
    data1 data2 data3)))
    nil)
    

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42. query1 (thread (do-query-1 request))
    query2 (thread (do-query-2 request))
    query3 (thread (do-query-3 request))
    0
    data1 (1
    data2 (2
    data3 (

    (send-async-response request
    data1 data2 data3)
    3
    

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43. View Slide

44. View Slide

45. (defn google [query]!
    (let [results-chan (chan)!
    timeout-chan (timeout 100)]!
    (go (>! results-chan ((go (>! results-chan ((go (>! results-chan (! ! ! !
    (go-loop [i 0 results []]!
    (if (= i 3)!
    results!
    (recur (inc i)!
    (->>!
    (alt! [results-chan timeout-chan] ([v] v))!
    (conj results)))))))
    Shared timeout!
    

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46. Event loop w/Callbacks
    Promises/Futures/Tasks
    Rx (push based FRP)
    Coroutines & Generators
    Actors
    Communicating Sequential Processes (CSP)
    async keyword
    

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47. View Slide

48. var  tweets,  answers,  kaggleProfile;

     

    twitter.getTweetsFor("bmabey",  function  (result)  {

       tweets  =  result;

       somethingFinished();

    });

    stackOverflow.getAnswersFor("bmabey",  function  (result)  
    {

       answers  =  result;

       somethingFinished();

    });

    kaggle.getProfileOf("bmabey",  function  (result)  {

       kaggleProfile  =  result;

       somethingFinished();

    });
    var  finishedSoFar  =  0;

     

    function  somethingFinished()  {

       if  (++finishedSoFar  ===  3)  {

           ui.show(tweets,  answers,  kaggleProfile);

       }

    }
    http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript
    

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49. var  tweets,  answers,  kaggleProfile;

     

    twitter.getTweetsFor("bmabey",  function  (result)  {

       tweets  =  result;

       somethingFinished();

    });

    stackOverflow.getAnswersFor("bmabey",  function  (result)  
    {

       answers  =  result;

       somethingFinished();

    });

    kaggle.getProfileOf("bmabey",  function  (result)  {

       kaggleProfile  =  result;

       somethingFinished();

    });
    var  finishedSoFar  =  0;

     

    function  somethingFinished()  {

       if  (++finishedSoFar  ===  3)  {

           ui.show(tweets,  answers,  kaggleProfile);

       }

    }
    http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript
    

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50. var  tweets,  answers,  kaggleProfile;

     

    twitter.getTweetsFor("bmabey",  function  (result)  {

       tweets  =  result;

       somethingFinished();

    });

    stackOverflow.getAnswersFor("bmabey",  function  (result)  
    {

       answers  =  result;

       somethingFinished();

    });

    kaggle.getProfileOf("bmabey",  function  (result)  {

       kaggleProfile  =  result;

       somethingFinished();

    });
    var  finishedSoFar  =  0;

     

    function  somethingFinished()  {

       if  (++finishedSoFar  ===  3)  {

           ui.show(tweets,  answers,  kaggleProfile);

       }

    }
    - Continuation Passing Style
    - Destroys readability
    - Forces shared mutable state
    - Can’t use normal language
    constructs for error handling and
    looping.
    http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript
    

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51. (go!
    (let [results (chan 3)]!
    (twitter/get-tweets "bmabey" c)!
    (stack-overflow/get-answers "bmabey" c)!
    (kaggle/get-profile "bmabey" c)!
    (send-async-response (async/into [] results)))
    

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52. View Slide

53. Q.all([

       twitter.getTweetsFor("bmabey"),

       stackOverflow.getAnswersFor("bmabey"),

       kaggle.getProfileOf("bmabey")  
    ]).then(function  (results)  {

       console.log(results[0],  results[1],  results[2]);

    });
    http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript
    

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54. Q.all([

       twitter.getTweetsFor("bmabey"),

       stackOverflow.getAnswersFor("bmabey"),

       kaggle.getProfileOf("bmabey")  
    ]).then(function  (results)  {

       console.log(results[0],  results[1],  results[2]);

    });
    http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript
    + Promises are values
    + Composable
    + Better error handling
    - Continuation Passing Style!
    - Still can’t use language constructs.
    - Mutable state still flying around.
    - By design, are not for streams!
    - Have to create a new promise
    for each event.. every time!
    !
    

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55. C# Async
    async void Go() {!
    _button.IsEnabled = false;!
    string[] urls = "clojure.org www.albahari.com/nutshell/
    golang.org".Split();!
    int totalLength = 0;!
    foreach (string url in urls)!
    {!
    var uri = new Uri ("http://" + url);!
    byte[] data = await new WebClient().DownloadDataTaskAsync (uri);
    _results.Text += "Length of " + url + " is " + data.Length +!
    totalLength += data.Length;!
    }!
    _results.Text += "Total length: " + totalLength;!
    }
    

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56. C# Async
    async void Go() {!
    _button.IsEnabled = false;!
    string[] urls = "clojure.org www.albahari.com/nutshell/
    golang.org".Split();!
    int totalLength = 0;!
    foreach (string url in urls)!
    {!
    var uri = new Uri ("http://" + url);!
    byte[] data = await new WebClient().DownloadDataTaskAsync (uri);
    _results.Text += "Length of " + url + " is " + data.Length +!
    totalLength += data.Length;!
    }!
    _results.Text += "Total length: " + totalLength;!
    }
    + No more Continuation Passing Style!
    - Still can’t use for streams.
    

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57. Async Adoption
    2007 - F# adds introduces async workflows
    2010? - C# adds async
    2012-13? - Scala library adds async via macros
    2013 - Python announced it will be adding it
    2014 - Javascript, ES7 proposal for async
    

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58. View Slide

59. (defn batch-until-pause!
    "Batches values from the `in` channel into a single a vec of values up until no!
    messages have appeared in `timeout-pause` milliseconds. `max-batch-time` is the!
    number of milliseconds to wait overall before sending a batch."!
    ([timeout-pause max-batch-time in]!
    (batch-until-pause timeout-pause max-batch-time in (chan)))!
    ([timeout-pause max-batch-time in out]!
    (go!
    (loop [batch []!
    batch-timeout (timeout max-batch-time)]!
    (alt!!
    ;; batch timed out, send what we have now and start a new batch timeout!
    batch-timeout (do!
    (>! out batch)!
    (recur [] (timeout max-batch-time)))!
    in ([v]!
    (if v!
    ;; we got a value, add it to the batch!
    (recur (conj batch v) batch-timeout)!
    ;; we got nil so the 'in' channel was closed, send our batch!
    (>! out batch)))!
    !
    ;; a timeout happened, send the batch and start a new one!
    (timeout timeout-pause) (do!
    (>! out batch)!
    (send-batch!)!
    (recur [] batch-timeout))))!
    (close! out))!
    out))
    

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60. What about Rx?
    

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61. What about Rx?
    + Very composable
    + Handles streams
    - Push-based semantics only cover ~80% use cases
    - Have to learn all the combinators
    - Writing a combinator can be difficult.
    - Mutable state with call backs again!
    !
    Awesome abstraction, but in the end it is more complex
    and less powerful than CSP.
    

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62. View Slide

63. View Slide

64. We’ve covered brain dead
    concurrency… what about
    mind blowing concurrency?
    

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65. View Slide

66. View Slide

67. (defn start-phone-process [phone]!
    (let [...]!
    (go-loop []!
    (alt!!
    off-hook (incoming-call ...!
    ....!
    (recur))))!
    !
    !
    !
    !
    !
    !
    !
    !
    !
    !
    !
    !
    

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68. (defn start-phone-process [phone]!
    (let [...]!
    (go-loop []!
    (alt!!
    off-hook (incoming-call ...!
    ....!
    (recur))))!
    !
    !
    (defn get-number [phone]!
    (let [...]!
    (go-loop [numbers []]!
    (alt!!
    on-hook ::hang-up!
    digit-pressed ([num] (let [numbers* (conj numbers num)]!
    (if (= numbers* 9)!
    ((recur numbers*))))!
    ...!
    ))))
    

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74. Differences with
    Actors…
    

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75. Macro Limitations
    

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76. Thank you!
    !
    !
    BenMabey.com
    !
    github.com/bmabey
    !
    @bmabey
    !
    

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